This invention relates to apparatus for separating magnetisable particles from a fluid in which they are suspended.
Known apparatus for separating magnetisable particles from a fluid in which they are suspended essentially comprises a separating chamber of non-magnetisable material and a magnet for establishing within the separating chamber a zone of high magnetic field intensity. The separating chamber has a fluid inlet and a fluid outlet and is loosely packed with a fibrous or particulate ferromagnetic material in order to provide within the chamber, when acted upon by the magnet, a large number of points of high magnetic field intensity separated by regions of lower intensity so that the local magnetic intensity within the separating chamber changes rapidly with distance. In operation a slurry containing a mixture of particles of relatively high magnetic susceptibility and particles of relatively low magnetic susceptibility is passed through the separating chamber from the inlet to the outlet thereof whilst a high intensity magnetic field is established in the region of the chamber, so that the particles of relatively high magnetic susceptibility are magnetised and attracted to, and retained on, the ferromagnetic packing material and in this way a separation of the particles of relatively high magnetic susceptibility from the particles of relatively low magnetic susceptibility can be achieved.
In order to establish a field of sufficient intensity in the region of the separating chamber and its packing material, the separating chamber is conveniently positioned in the zone of highest intensity of a magnetic field generated by a magnet. An arrangement which has been found to be very suitable in practice is to place a separating chamber of cylindrical shape within the central bore of an electromagnet coil in the form of a solenoid. In this arrangement the lines of magnetic force are generally parallel to the longitudinal axis of the coil and thus of the separating chamber. One disadvantage of this arrangement is that, although the magnetic field is substantially parallel to the longitudinal axis near the middle of the length of the coil, near the ends of the coil the field tends to "fan out" with consequent reduction in field intensity in these regions and dissipation of energy. As a result the average magnetic field intensity within the separating chamber is lowered and the separation efficiency of the separating chamber is decreased. The tendency for the field to fan out at the ends of the coil may be at least partially corrected by providing the coil with extra turns at its ends to increase the intensity of the field in these regions. This solution is, however, expensive especially when the electromagnet is of the superconducting type and the whole of the conductor constituting the coil, including the extra turns at the ends, must be cooled to a temperature which is little higher than absolute zero. Also, the provision of extra turns at the ends of the coils of a superconducting electromagnet, with a consequent increase in the outer diameter of the coil, would present problems in the design of the cryogenic apparatus which would be difficult and expensive to overcome.